1. Field of the Invention
The present invention relates to agricultural paper and also to a process for producing the same. More particularly, it relates to paper suitable for the production of pots for raising seedlings and also to a process for producing the same. The pots made from the agriculatural paper of the invention permits the seedlings to take root through the wall of the pots after the seedlings are transplanted to the field together with the pots.
2. Description of the Prior Art
It has been common practice to raise seedlings in paper pots and transplant the raised seedlings to the field together with the pots. This cultivation method saves labor and increases the yield of crops.
The paper pot to be used as mentioned above should ideally have the following characteristic properties: (1) Air and water permeability necessary for the growth of seedlings; (2) Adequate separation between pots so as to prevent the roots of seedlings in adjacent pots from getting entangled together in the period of raising seedlings; (3) Strength and stiffness of sufficient paper to hold seedlings and soil in the pot while transplantation is being carried out; (4) Ability to resist rotting caused by various types of soil in the period of raising seedlings; and (5) Structure that permits the roots of crops to penetrate the pot wall after transplanation. In addition, the paper pots should be inexpensive because they cannot be recovered for reuse after transplantation.
To meet the above-mentioned requirements, there have been proposed various kinds of agricultural paper. For example, Japanese Patent Laid-open No. 150249/1977 discloses agricultural paper composed of special paper or paperboard and nonwoven fabric bonded to both sides thereof with a paste or by the other means, said paper or paperboard rotting or decomposing in soil. The nonwoven fabric is made of synthetic fiber, rots slowly in soil, and has a highly porous reticulated structure. Japanese Patent Laid-open No. 26118/1981 discloses agricultural paper composed of first and second thin films bonded to each other with a water-soluble paste, said second thin film being resistant to rotting and porous enough for radicles to penetrate. Japanese Patent Laid-open No. 109524/1981 discloses agricultural paper composed of paper of natural fiber and synthetic fibers uniformly dispersed thereon and bonded thereto, the amount of said synthetic fiber being 5-20 g/m.sup.2. Japanese Patent Laid-open No. 60919/1983 discloses agricultural paper composed of paper of natural fiber and a thin film of composite synthetic fibers (less than 15 g/m.sup.2) bonded to each other, said composite synthetic fiber being composed of a core made of a synthetic resin having a higher melting point and a sheath made of a synthetic resin having a lower melting point.
In the case of the conventional two- or three-layered agricultural paper as mentioned above, the layer of synthetic fiber should be as the thin and porous as possible so that it permits the growth of radicles when the paper layer made of natural pulp has rotted after transplantation. However, according to the conventional technology, it is very difficult to produce a uniform, porous, thin film having a basis weight lower than 10 g/m.sup.2. It is also difficult to uniformly bond such a thin film to the paper layer made of natural pulp. Therefore, the conventional agricultural paper was inevitably expensive.
Another disadvantage of the conventional agricultural paper is that the paper layer made of natural pulp and the layer of artificial or synthetic thin film separate easily from each other on contact with water in the period of raising seedlings, if the two layers are bonded with a paste. When the two layers are separated from each other, the pot does not hold the soil and seedlings completely during transplantation by a machine. This cause problems in the transplantation.